Many flashlights can be easily converted to use white LEDs instead of regular incandescent bulbs. The flashlight can quickly be returned to its original bulb and battery configuration later if you wish. Of course this procedure will void the warranty on your flashlight, but the only permanent modification made is increasing the size of the hole in the reflector assembly.

• Take off the flashlight head and remove the incandescent bulb. Save the bulb for future use if you ever decide to return your flashlight to its original condition.
• Trim the tabs off of the LED leads using nippers. A flat needle file or sandpaper can help smooth the lead.
• Trim the LED leads off to 1/4 inch in length.
• Insert the LED into the bulb socket. If does not light up, reverse the LED and it will. If not, check your batteries, and also make sure the LED leads are not touching each other.
• Remove the reflector, and using a 1/4 inch drill bit carefully widen the hole for the bulb to 1/4 inch. Our LED bulbs do not need reflectors since they emit light at a set 20° angle, but the reflector assembly is needed in a MiniMaglite® for the switch to function properly.
Now try your flashlight with the original AA batteries. It should light up, but somewhat dimmly. If you run it like this, the batteries will last for weeks of continous on-time. It's not very bright, but it is enough light to find the keyhole or make your way to the bathroom. To run the LED at full brightness you'll need to use 3 N cells.
Next,
• Install the 3 N-cell batteries.
• Install a spacer (containing an internal current-limiting resistor)
You'll need to build a spacer to hold the resistor and make the whole battery pack come out to the right length. Our prototype used a 3/8 inch length of 1/2 inch diameter wooden dowel . We drilled short holes at the ends of the dowel for battery contacts made of small machine screws. We then drilled an off-center hole all the way through the spacer for the resistor, and wrapped each resistor lead around the contacts.

This conversion was a quick, easy project. The LED version of this flashlight gives over 100 hours of bright, broad light from a full 12 volt NiMH battery pack. We started with a 6-LED circuit using 2 parallel strings of 3 white LEDs, each string with its own 50 Ohm resistor. I simply used a voltmeter to determine which lamp terminals were positive and negative, and soldered the leads from the LED circuit board to the normal lamp terminals. The LED assembly is held in place on the concave surface of the reflector with dabs of epoxy .

Larger flashlights can also be converted.  We are currently experimenting with some different versions of this project. These conversions also need a current-limiting resistor in the circuit. Our experiments have shown that a 15 ohm 1/4 watt resistor is about right for this...it gives an LED current of around 30 milliamps when the batteries are fresh. Because of the extra power available from these larger batteries, it is possible to mount 2 or 3 white LEDs at once, increasing light output. We are currently experimenting with this by breaking the glass of a burnt-out bulb and soldering in 3 LEDs. The easiest way to install the resistor is to use a 4-cell flashlite with only 3 cells, plus a dummy spacer that contains the resistor. More on this project soon!
It was a dark and stormy night. We couldn't find a flashlight anywhere in the house, the kids probably lost all of them in their rooms...and something was on the roof! It sounded big, possibly a mountain lion or a bear. So I dug up an old NiCad computer battery, gave it a 5-minute quick charge at 1000 milliamps, took a spring clamp and clipped 3 LEDs directly to the battery. Let there be light! It was only a packrat, and of course this story is mostly fabrication, but the resulting flashlight was pretty neat .